1. Field of the Invention
The present invention relates to a liquid crystal display device for use in office automation (OA) equipment (e.g., word processors and personal computers), portable information equipment (e.g., electronic books), video cassette recorders (VCRs) incorporating a liquid crystal monitor, and the like.
2. Description of the Related Art
Recently, liquid crystal display devices have been widely used in OA equipment (e.g., word processors and personal computers), portable information equipment (e.g., electronic books), video cassette recorders (VCRs) incorporating a liquid crystal monitor and the like, utilizing the features of a thin display thickness and low power consumption.
Such liquid crystal display devices include a transmission-type liquid crystal display device using a thin, transparent, electrically conductive film such as ITO (Indium Tin Oxide) as pixel electrodes, and a reflection-type liquid crystal display device using reflective electrodes of, for example, a metal as pixel electrodes.
Unlike CRTs (cathode ray tubes) and EL (electroluminescence), liquid crystal display devices are not self-light-emitting display devices. Therefore, in the case of the transmission-type liquid crystal display device, an illumination device such as a fluorescent tube, which is a so-called backlight, is placed behind the liquid crystal display device, whereby a display is provided using incident light of the backlight. On the other hand, in the case of the reflection-type liquid crystal display device, a display is provided by reflecting incident light from the outside by the reflective electrodes.
Since the transmission-type liquid crystal display device uses the backlight to provide a display, the transmission-type liquid crystal display device has an advantage of providing a bright, high-contrast display without being significantly affected by the brightness around the liquid crystal display device. However, the backlight consumes about 50% or more of the overall power consumption of the liquid crystal display device, whereby power consumption is disadvantageously increased.
On the other hand, since the reflection-type liquid crystal display device does not use such a backlight, the reflection-type liquid crystal display device has an advantage of significantly reducing power consumption. However, brightness and contrast of the display are affected by environmental factors such as brightness around the liquid crystal display device and/or the conditions under which the liquid crystal display device is used.
In the case of the reflection-type liquid crystal display device, visual recognition of the display is affected by environmental factors such as brightness around the liquid crystal display device, and is extremely deteriorated particularly when ambient light is dark. On the other hand, in the case of the transmission-type liquid crystal display device, visual recognition of the display is reduced when ambient light is extremely bright such as in good weather.
As means for solving such problems, liquid crystal display devices having functions of both reflection- and transmission-type liquid crystal display devices (hereinafter, referred to as "transmission/reflection-type liquid crystal display devices") are disclosed in copending U.S. application Ser. No. 09/122,756 filed on Jul. 27, 1998 and U.S. application Ser. No. 09/220,792 (not yet assigned; first named inventor Masumi KUBO et al.; assigned to Sharp Kabushiki Kaisha; Sharp internal reference No. 98435/US/PB; Attorney Docket No. 829-493) filed on Dec. 28, 1998, which is a continuation-in-part of U.S. application Ser. No. 09/122,756. These U.S. applications are incorporated herein by reference.
In the transmission/reflection-type liquid crystal display devices proposed by the above-mentioned U.S. applications, each pixel region includes a reflective electrode region for reflecting ambient light and a transmissive electrode region for transmitting light from a backlight, the transmissive electrode region being formed from a film having a relatively high light-reflectance. As a result, the transmission/reflection-type liquid crystal display device (i) serves, in a pitch-dark environment, as a transmission-type liquid crystal display device which provides a display by using light from the backlight transmitted through the transmissive electrode regions, (ii) serves, in a dark environment, as a transmission/reflection-type liquid crystal display device which provides a display by using both light from the backlight transmitted through the transmissive electrode regions and ambient light reflected by the reflective electrode regions, and (iii) serves, in a bright environment, as a reflection-type liquid crystal display device which provides a display by using light reflected from the reflective electrode regions.
Hereinafter, the terms "reflective electrode region", "transmissive electrode region", "reflective region" and "transmissive region" as used herein will be defined.
A reflection-type liquid crystal display device which provides a display in a reflection mode by using ambient light has reflective electrode regions for reflecting ambient light transmitted through a liquid crystal layer, the reflective electrode regions being provided on one of a pair of substrates. The reflective electrode region may be formed from a reflective electrode, or may be formed from a combination of a transparent electrode and a reflective layer (reflective plate). In other words, an electrode for applying a voltage to the liquid crystal layer may be formed from the transparent electrode, and the reflective layer for reflecting incident light does not have to function as an electrode.
Like the reflection-type liquid crystal display device, the transmission/reflection-type liquid crystal display device also has reflective electrode regions, in addition to transparent electrode regions. Each reflective electrode region may be formed from a reflective electrode, or may be formed from a combination of a transparent electrode and a reflective layer (reflective plate). Each transparent electrode region is typically formed from a transparent electrode.
In the transmission/reflection-type liquid crystal display device according to the present invention, a region for providing a display in a transmission mode is referred to as a transmissive region, whereas a region for providing a display in a reflection mode is referred to as a reflective region. The transmissive region includes a transmissive electrode region and a liquid crystal region defined by the transmissive electrode region, and the reflective region includes a reflective electrode region and a liquid crystal region defined by the reflective electrode region. Although a semi-transmission/reflection-type liquid crystal display device using a semi-transmissive/reflective film (i.e., a porous, reflective film) has reflective electrode regions and transmissive electrode regions, light passing through respective liquid crystal regions defined by the reflective electrode regions and transmissive electrode regions is mixed and overlaps each other. Therefore, a region for providing a display in a transmission mode (i.e., a transmissive region) and a region for providing a display in a reflection mode (i.e., a reflective region) cannot be defined independently. In other words, among the liquid crystal display devices wherein each pixel region has a transmissive electrode region and a reflective electrode region, a liquid crystal device in which a region for providing a display in a transmission mode and a region for providing a display in a reflection mode cannot be defined independently (that is, they substantially overlap each other) is referred to as a semi-transmission/reflection type liquid crystal display device.
The term "pixel region" as used herein will now be described. The liquid crystal display device of the present invention includes a plurality of pixel regions for providing a display. A single pixel region indicates a portion (component) of the liquid crystal display device, which constitutes a pixel, i.e., a minimum unit of the display. Typically, in an active matrix-type liquid crystal display device having a counter electrode and a plurality of pixel electrodes which are switched by respective active devices (e.g., thin film transistors (TFTs)) formed in a matrix, each pixel region includes a respective pixel electrode, a counter electrode region facing the pixel electrode, and a liquid crystal region located therebetween. In a simple matrix-type (or passive matrix-type) liquid crystal display device having stripe-shaped electrodes (scanning electrodes and signal electrodes) which are formed on respective substrates so as to intersect each other with a liquid crystal layer interposed therebetween, a pixel region includes respective intersection regions of the corresponding scanning and signal electrodes and a liquid crystal region located at the intersection. In the transmission/reflection-type liquid crystal display device of the present invention, each pixel electrode has a reflective electrode region and a transmissive electrode region.
In a color display device, color filter regions are formed so as to correspond to the respective pixel regions, and light passing through the color filter regions is controlled at the respective pixel regions, thereby providing color display by an additive color mixing method. Typically, a single color picture element region is formed from three pixel regions: red pixel region (R-pixel region), green pixel region (G-pixel region) and blue pixel region (B-pixel region). A color filter region of a color filter layer which corresponds to the respective pixel region of a color filter layer, and indicates, for example, a red filter region (R-filter region), a green filter region (G-filter region) or a blue filter region (B-filter region). A color filter region is a portion of a respective color layer (i.e., red layer (R-layer), green layer (G-layer) or blue layer (B-layer)). A layer including a plurality of color layers is herein referred to as a color filter layer. In the case where the plurality of color layers are arranged in stripes, the color filter layer includes a plurality of R-layers, G-layers and B-layers which are arranged in a cyclic manner (i.e., RGBRGB . . . ). The color filter layer may include a light-shielding layer (black mask) provided between the color layers or between the color filter regions. The color filter layer includes a plurality of color layers of different colors, each color layer having color filter regions provided so as to correspond to a plurality of pixel regions. A region having no color layer, which is provided in each pixel region, is herein referred to as a non-color filter region. In the present invention, the color filter layer includes color filter regions and non-color filter regions.
It should be noted that the term "retardation" as used herein indicates retardation with respect to light which is incident perpendicularly to a liquid crystal layer or phase compensation element (e.g., quarter-wave plate or half-wave plate) unless otherwise specified.
The above-mentioned reflection/transmission-type liquid crystal display device can always provide excellent visual recognition of display regardless of the brightness of ambient light. However, in the case where a conventional color filter is used for color display, optimum color display cannot be obtained both in a transmission mode and reflection mode.
FIG. 30 is a plan view showing the case where a normal color filter layer 24 as conventionally used is provided in the above-mentioned reflection/transmission-type liquid crystal display device. As shown in FIG. 30, the color filter layer 24 includes color filter regions 24A, 24B and 24C. Each of the color filter regions 24A, 24B and 24C is a part of a respective stripe-shaped color layer (i.e., R-layer, G-layer or B-layer), and is formed so as to entirely cover a respective pixel region having a reflective electrode region (R region) 3 and a transmissive electrode region (T region) 8.
In the case where such a conventional color filter layer 24 is applied to the above-mentioned reflection/transmission-type liquid crystal display device as shown in FIG. 30, light from the backlight is transmitted through a color filter layer in a transmissive region only once, whereas ambient light is transmitted through a color filter layer in a reflective region twice due to the reflection. Therefore, in the case where the same color filter is used both in the transmissive region and reflective region, display in the reflective region becomes dark. For example, a transmittance of a color filter for use in a normal transmission-type liquid crystal display device is about 30% after human eye's color sensitivity collection, while being about 16% when this color filter is used in a reflection-type liquid crystal display device.
Japanese Laid-Open Publication No. 8-286178 discloses a liquid crystal display device which realizes bright, high color-purity color display, wherein a color filter has an island-shaped color portion in each pixel, and an opening (i.e., a region having no color portion) is provided around each color portion. However, Japanese Laid-Open Publication No. 8-286178 merely discloses the structure of the color filter layer for use in a transmission- or reflection-type liquid crystal display device, and fails to disclose an optimal structure of a color filter layer for use in a transmission/refection-type liquid crystal display device including, in every pixel region, both a reflective electrode region for reflecting ambient light and a transmissive electrode region for transmitting light from the backlight. In other words, Japanese Laid-Open Publication No. 8-286178 falls to disclose features and arrangement of color filter region (i.e., color portion) and non-color filter region (i.e., opening). When the technique of forming a color filter as disclosed in Japanese Laid-Open Publication No. 8-286178 is applied to such a transmission/reflection-type liquid crystal display device, only a light-colored, low color-purity display can be obtained. Therefore, it is very difficult to realize a color filter capable of providing a bright, high color-purity color display both in the transmissive and reflective regions. Moreover, in the case where a substrate having a color filter layer and a substrate having pixel electrodes are laminated to each other so as to be shifted from each other, that is, in the case where the substrates are mis-aligned with respect to each other, the area of each non-color filter region may vary, thereby reducing display quality.